Hola,
Si, depende de la "localizacion" de tu servidor PHP. En
http://www.php.net/manual/en/pcre.pattern.syntax.php :
Cita: A "word" character is any letter or digit or the underscore character, that is, any character which can be part of a Perl "word". The definition of letters and digits is controlled by PCRE's character tables, and may vary if locale-specific matching is taking place (see "Locale support" above). For example, in the "fr" (French) locale, some character codes greater than 128 are used for accented letters, and these are matched by \w.
Y en strtolower() (
www.php.net/strtolower) tambien lo comentan.
Puedes probar a usar setlocale() (
www.php.net/setlocale). Lo que ya no se si depende del sistema operativo del servidor. Si no te funciona ponte en contacto con tu hosting.
Saludos.